Apparatus for submerged combustion heating of liquids



Jan. 15, 1963 R. SWITZER ETAI. 3, 8

APPARATUS FOR SUBMERGED COMBUSTION HEATING OF LIQUIDS Filed Aug. 25,1959 2 Sheets-Sheet 1 F550 A/QU/D Jall- 1963 R. L. SWITZER ElAl.3,073,683

APPARATUS FOR SUBMERGED COMBUSTION HEATING OF LIQUIDS Filed Aug. 25,1959 2 Sheets-Sheet 2 R. 4. smrzsx? E 4 we. LIEFFEIQS CLYD ERG 4rr meweUnited States Patent 3,073,683 APPARATUS FOR SUBMERGED COMBUfiTlGHEATING 0F LIQUIDS Robert L. Switzer, Long Beach, William C. Liefiers,Garden Grove, and Clyde H. 0. Berg, Long Beach, Calif, assignors, bymesne assignments, to Collier Carbon and Chemical Corporation, acorporation of California Filed Aug. 25, 1959, Ser. No. 835,946 13Claims. (Cl. 23-275) 1 formation of stable phosphoric acid aerosols inthe combus gases.

Submerged combustion heating is a process wherein a fuel is burnedwithin a chamber and the resultant hot combustion gases are dischargedbeneath the surface of a body of liquid being heated, transferring theirheat to the cooler liquid by direct contact. After passage through theliquid the gases are usually discharged to the atmosphere as exhaustgases. The flame within the burning chamber, which in most, but not allinstances is partially or wholly submerged in the liquid, is usuallyprevented from impinging upon the liquid by extending the burningchamber walls or by attaching an extended conduit, usually referred toas a dip tube, to the burning chamber. To maintain proper burningconditions within the chamber, it has been customary to constrict theoutlet of the burner wall extension or dip tube.

From the preceding discussion, it is apparent that submerged combustionheating requires fewer heat transfer steps than do the more conventionalindirect heat I transfer methods, and that, therefore, submergedcombustion is more eflicient than the commonly employed heatingtechniques. Despite this, submerged combustion has found only limitedacceptance since its introduction in the latter years of the lastcentury; its use generally confined to heating corrosive or scaledepositing liquids which can not be satisfactorily heated byconventional indirect heat transfer methods.

Despite prior limited acceptance of submerged combustion heating, thisimprovement thereof is applicable for the heating of any liquid,solution, or slurry, however, it is particularly suited for the heatingof those liquids which tend to deposit excessive amounts of solids onthe heating equipment during the heating step. Examples'of such liquidsare aqueous salt solutions, for instance, aqueous solutions of thecarbonates, sulfates, sulfites, nitrates, phosphates and halogen saltsof sodium, potassium, calcium, magnesium, iron, aluminum, ammonium, etc.Acids, such as sulfuric and phosphoric, can also be heated in accordancewith the invention. Solutions of organic compounds, such as sugar,starch, and rubber solutions, can also be heated, as well as slurries ofsolids, such as slurries of waste citrus products or the liquor slurriesof wood chips encountered in the paper industry. The heating of thesolutions can be for any purpose, for instance, to raise the liquidtemperature, to supply reaction heat or to supply the latent heat ofvaporization of a solvent to concentrate the solution. This invention isparticularly suited for supplying heat to evaporating solutions, sinceit is in this type of operation that scale and deposit formationfrequently occurs.

Submerged combustion has previously been used to heat scaleformingliquids; however, its use in this service re quires frequentinterruption to remove solid deposits "Ice The use of submergedcombustion to supply heat in I the evaporative concentration ofphosphoric acid has also been suggested; however, previous investigatorshave reported that its use in this service is not satisfactory sincephosphoric acid aerosols are formed in the exhaust gases. These aerosolsare exteremely minute particles of phosphoric acid, 0.2 to 2 microns indiameter, which cannot be removed by conventional absorption orscrubbing steps. Since the aerosol comprises corrosive phosphoric acidand since it forms a visible White fume, the venting of the exhaustgases to the atmosphere is objectionable and expensive aerosol removalsteps must be taken. When wet process phosphoric acid, i.e., thatproduced by leaching phosphate rock with sulfuric acid, is heated, thefluorine compounds which are present as impurities in the acid areviolatilized and are absorbed by the phosphoric acid aerosol therebyincreasing its corrosivity. Other impurities in the crude acid, e.g.,organic matter and salts of aluminum, iron, magnesium, as well asresidual calcium sulfate, cause frothing and foaming of the acid duringheating and in part cause solid deposition on heating surfaces.Accordingly, submerged combustion has not been widely accepted as amethod for supplying heat to the evaporative concentration of phosphoricacid.

It is a purpose of this invention to provide a submerged combustionheating apparatus which, when used to heat a deposit-forming liquid,does not require frequent clean- It is also a purpose of this inventionto provide a method and apparatus to concentrate dilute phosphoric acidby evaporation while supplying the necessary heat with an improvedsubmerged combustion technique which prevents formation of objectionableamounts of phosphoric acid aerosols in the exhaust gases.

The aforementioned purposes are achieved by use of an improved submergedcombustion apparatus described as follows:

A submerged combustion burner consisting of a com bustible gas supplyconduit, suitable ignition means, and a burning chamber, isconcentrically disposed within a surrounding conduit, hereafter referredto as a dip tube,

' which extends a substantial distance below the burning substantiallygreater diameter which also concentrically surrounds the dip tube.

In operation, a pre-mixed stream of combustible fluid with an oxidant,preferably natural gas and air, are supplied to the burning chamber,ignited, and the resultant hot combustion gases flow from the burningchamber into the surrounding dip tube. These hot gases fiow downwardlythrough the dip tube and pass through a special nozzle into contact withthe cold liquid. The mixed liquid and gas stream reverses the directionand flows upwardly through the annulus between the dip tube and thesurrounding subjacent conduit. This annulus is sized sufiiciently smallin area in relation to the gas and liquid flow rates so that the gasexerts a. lifting effect on the liquid. The mixed gas and liquid streamdischarges from the upper open end of the subjacent conduit into thelarger diameter superimposed conduit. In this latter conduit the gas andliquid separate. The gas is removed overhead and the liquid flows backinto the mixed gas-liquid stream within the annulus. A continuous bleedstream of liquid is removed as product from the upper region of theannulus. The apparatus thus permits repeated contact between the liquidand hot gas and thereby insures a high rate of heat transfer.

The specially designed nozzles of this invention constrict the lower endof the dip tube and are provided with means to remove or prevent theformation of solid deposits across the constriction orifice. Thesenozzles are essential to the proper functioning of the apparatus sincethe use of an open-ended dip tube, i.e., without a nozzle, results inliquid surging which interferes with good flame conditions and whichsplashes liquid .on the hot inside surfaces of the dip tube. This liquidis dried in place by the hot gases to form a ,hard hygroscopic solid,while the volatilized fraction collects in the upper portion of the diptube. When Wet process phosphoric acid is being heated, the volatilizedfraction contains fluorine and phosphorous compounds which create aserious corrosion problem. The use of a nozzle avoids this difficulty;however, if the nozzle does not have means to remove or prevent theaccumulations of solid deposits, it is not satisfactory because depositsform on the constricted flow area of the nozzle and greatly disturb theflow pattern, creating excessive turbulence and eddy currents whichcause the dispersion of phosphoric acid aerosols in the combustiongases.

The nozzles suitable for this invention can have single or pluralpassageways for the hot gases. These passageways may simply be holesbored through an end plate on the dip tube, but preferably their shapeapproaches that of a venturi, with a smoothly converging entrance, anarrow throat and a smoothly diverging exit, so that eddy currents andhigh turbulence which contribute to aerosol formation are avoided. Whenthus shaped, the entrance and exist passageways can suitably take atrumpet, conical or conoidal shape. If conical in shape, the total anglebetween the entrance walls is not greater than 40 to 50 degrees andpreferably between about 25 to 30 degrees. The angle between the exitpassageway walls, also if conical, is no greater than 30 to 40 degreesand preferably less than about degrees. Regardless of the specific shapeof the passageways, the convergence and divergence of the passageway isgradual, with a smooth transition through the throat so as to avoid theformation of a vena contracta, i.e., the maximum contraction of astream, normally formed downstream from a sharp edge orifice or animproperly designed nozzle.

The removal and/ or prevention of solids accumulation on the nozzle flowpassageways is essential to avoid excessive turbulence with resultingaerosol formation. Following is a brief discussion of nozzle embodimentsof this invention which achieve this goal:

To prevent the liquid from adhering to the nozzle surfaces andevaporating to. dryness in place, the nozzle can be coated with arepellant for. the liquid being heated. For aqueous solutions, thiscoating can be of any suitable water-repellant material, such aspolymers of tetrafluoroethylene, trifiuorochloroethylene, or vinylchloride, or copolymers of vinyl chloride with vinyl acetate orvinylidene chloride. The first of the aforementioned coatings isavailable under the trade name Teflon from E. I. du Pont de Nemours &Co. These coatings can be employed alone, or can be combined with any ofthe following nozzle modifications to avoid solid deposition.

Cooling the nozzle keeps the latter free of deposits and this cooling isachieved in this invention by thecirculation of a gaseous or liquidcooling medium in indirect contact with the fouling surfaces. Indirectcontact is achieved by circulating the cooling medium through a chamberhollowed in the nozzle body to form an annulus perpendicular to andsurrounding the single or plural longitudinal gas flow passagewaysthrough the nozzle. Heat insulation is provided between the coolingfluid and those walls of the nozzle which do not normally in cur soliddeposition and which, therefore, do not need to be cooled.

Direct contact between a cooling liquid and the fouling surface of thenozzle can be achieved with the same nozzle construction as describedabove, if the nozzle body is constructed of a material which ispermeable to the flow of the fluid. Permeation of the cooling fluid toparts of the nozzle other than the fouling surfaces, i.e., those whichnormally incur solid deposition, can be prevented by sealing theappropriate walls of the cooling fluid annulus with a non-permeableresinous sealing material. When a liquid is chosen as the coolingmedium, additional cooling of the surface is achieved by evaporation ofthe liquid from the surface into the hot combustion gas stream. When thecooling liquid is also a solventfo r thedeposits, loosening and removalof the de posits is augmented by the dissolving power of the liquid.

Rather than constructing the nozzle with a hollowed interior for theflow of wash or cooling fluids, the nozzle can be solid and scrapingmeans can be provided to remove solids from the surfaces of thelongitudinal passageways. In this embodiment a bob with an invertedtruncated cone shape is placed with its small diameter end within eachlongitudinal passageway and a bob stop is placed beneath each bob. Thebob can be mechan ically operated up or down or can float within thepassageway so that normal pressure fluctuations will move it up anddown, against the surfaces of the passageways, scraping away anydeposits. To prevent undue turbulence, the bob can be egg-shaped and canhave a spiral groove or a raised spiral thread on its surface to spin itand obtain a good scraping action.

The invention is more completely described by reference to the drawingsof which:

FIGURE 1 shows the submerged combustion heating assembly of thisinvention;

FIGURES 2 to 4 show the dip tube nozzle cooling and/or washingembodiments of this invention; and

FIGURE 5 shows the scraping embodiment of this invention.

Referring now to FIGURE 1, the heating apparatus consists of an uppermetal vessel 6 closed at its upper end by plate 4 and fitted at itslower end with an inclined bottom plate assembly 13, which supportsconcentrically disposed, subjacent vessel 15. Vessel 15 is closed at itslower end with plate 19 and is fitted with graphite liner 16 andgraphite plate 17. The graphite liner 16 at its upper end is threadedinto the tapered bottom of a second graphite liner 8 having a greaterdiameter than liner 16 and extending a substantial distance up thelength of vessel 6, but terminating below the entrance of conduit 5 intovessel 6. A concentric graphite dip tube 7 extends the length of vessel6, into graphite liner 16 and is provided with a special dischargenozzle 14, discussed in detail below, at its lower end which liesslightly above plate 17. A burner 9, comprising spark discharge ignitingmeans, not shown, and a fuel supply conduit 3 is positioned within diptube 7 and is sufficiently spaced above the latters lower end that theflame from the burner does not extend to nozzle 14. A liquid feed inlet18 extends into vessel 15 through base plates 19 and 17, and a liquidproduct outlet 10 is tapped through vessel 15 and liner 16 at an angleinclined about 15 degrees from the horizontal. A Teflon tube 11,attached to outlet 10, passes to the exterior of the heating assemblythrough conduit 12 in bottom plate assembly 13 to permit withdrawal ofthe heated product.

In the discussion of this apparatus, reference has been made to graphiteliners and plates. Graphite is suitably employed in this invention whenwet phosphoric acid is concentrated because it has sufficient corrosionresistance to the phosphorus and fluorine compounds present in thisacid. It is, of course, obvious that other-materials of construction maybe employed which are also inert to the liquid being heated. Whenpossible, with less-corrosive liquids, it is preferred to dispense withgraphite liner 16 and plate 18 and to construct liner 8 of metal. Insuch an embodiment, metal liner 8 has a tapered or inclined bottom whichis attached to and in open communication with the upper end of subjacentvessel 15. The corrosion resistant plastic tube 11 is also eliminatedand the heated liquid allowed to drain into vessel 6 through outlet tap10. It is also apparent that clip tube 7 may suitably be constructedfrom any material which is resistant to the liquid being heated.Although the use of a dip tube is preferred, it is of course apparentthat the function of the dip tube could also be accomplished byextending the walls of burner 9. Alternatively, dip tube 7 need notextend the length of vessel 6, but can be attached directly to the lowerend of burner 9.

In operation, a pre-mixed stream of natural gas and air is supplied tothe flame within burner 9, and the resulting hot combustion gases flowdownwardly through dip tube 7 and nozzle 14 into zone A where they comein direct contact with cold feed liquid supplied through conduit 18. Thecombustion gases reverse their flow and pass upwardly through annulus Binto enlarged zone C from whence they are removed via conduit 5. As thegases pass upwardly through annulus B they exert a lifting effect on theliquid and carry large amounts of the same up into the annulus, therebyprolonging the contact time between the liquid and gases and resultingin a high rate of heat transfer. In zone C, the liquid disengages fromthe gases and flows down the inclined bottom of liner 8 and down theinterior surface of liner 16. As the liquid passes outlet It), some ofit is removed asproduot and the remainder flows into repeated contactwith the hot combustion gases. Because of the turbulence within theannulus, some liquid is also thrown directly into outlet 10 and isremoved.

The positioning of outlet 16 in the upper portion of vessel I5 providesfor adequate contacting of the hot gases and the liquid. In addition,sizing the outlet sufficiently small provides adequate residence time ofthe liquid within the heating zone without the need of a level controlsystem which would be extremely difficult to use on the highly turbulentmixed gas-liquid phase in the annulus.

FIGURE 2 illustrates the construction of a burner nozzle modificationwhich successfully prevents the formation of solid deposits in the areaof restricted flow. In this modification, the lower end of clip tube 7is threaded on its inside surface and a doughnut shaped graphite plug 23is constructed to thread into the end of the dip tube. Plug 23 isthreaded at its lower end to receive a nozzle liner such as shown by 25,the interior surface of which comprises a longitudinal flow passagewayfor the hot combustion gases. The lower half of plug 23 has a greaterinternal diameter 36 than its internal diameter 27 in its upper portionso as to provide recessed surface 24. A groove 28 is cut into the upperend of enlarged diameter section 36 to provide shoulder 29. Flowpassageways 21 and 22 are bored as shown into opposite sides of plug 23.Bore 22 can, if desired, be tapped to receive the threaded end ofconduit Zll, or if desired, the bore diameter 22 can be chosen so as topermit a swedge fit of conduit within the hole. A ring 33 constructedfrom a suitable heat insulation ma erial, for instance Teflon, isinserted into plug 23 and held in place by the lower flared end ofnozzle liner 25 when the latter is threaded into the lower end of plug23. The upper edge of ring 33 is of slightly smaller diameter thangroove 28 into which it fits to allow for the dissimilar thermalexpansion of the ring and plug during use. The exterior surface ofnozzle liner 25 is recessed along a portion of its length as shown by 32to provide an enlarged annular chamber in the nozzle assembly. Ashoulder 31 is provided at approximately the central portion of theliner to support an O-ring seal which is compressed against shoulder 29of plug 23 when liner 25 is threaded into the plug, thereby sealing theannular space between liner 25 and plug 23. When the assembly iscomplete, recessed surface 32 of the liner and the interior surface ofring 33 form an annular chamber D which surrounds the longitudinal gaspassageway through the nozzle so that the circulation of a cooling fluidthrough conduits 20 will remove heat from the non-insulated surfaces 32and 35 of the nozzle liner. Recessed surface 32 is shown to extendupwardly to only approximately the middle of the liner, for it is inthis region that sol-id deposition is found to occur on the gaspassageway surface in an uncooled nozzle. However, it is clear that, ifdesired, a greater height of the liner could be cooled merely byextending the height of enlarged diameter section 36, ring 33 andrecessed surface 32. Preferably, the interior longitudinal gaspassageway surface is coated with a nonwetting material, for instanceTeflon, to decrease the tendcncy for solids to deposit on this surface.

The embodiment of FIGURE 2 may also be used when it is desired to washthe surface of liner 25. In this modification, liner 25 is constructedof a material which is permeable to the wash fluid, such as air orwater, which is circulated through chamber D. Suitable permeablematerials may be selected from permeable ceramics, or permeable metalsand glass which are made by sintering of metal or glass powders. Wherean extremely corrosive liquid, such as wet process phosphoric acid, isheated, the aforementioned materials are not suitable, and permeablegraphite should be employed. If it is desired to use only the indirectcooling feature with a graphite liner, surfaces 32 and 35 can be madeimpervious to the cooling fluid by coating with plastic or resinousmaterial. Since graphite has a high thermal conductivity, thelongitudinal gas passageway surface is readily cooled by thismodification.

FIGURES 3 and 4 depict a nozzle liner having a plurality of longitudinalgas passageways and provision for circulation of a cooling and/orwashing fluid. The elevation view, FIGURE 3, shows the liner/I2 to be asolid plug with a plurality of longitudinal venturi-shaped gaspassageways 43. The upper end of liner 42 is slightly less than diameter27 of plug 23 shown in FIGURE 2,

and the base of liner 42 is provided with a threaded edge 39 which fitsinto the threaded socket base of plug 23 to permit the liner to besecurely fastened within this plug. Shoulder 38 is provided on liner 42to support a sealing ring against shoulder 29 of plug 23 to seal theannular chamber formed between wall 40 of liner 42 and insulation ring33. During use, a fluid is circulated through 20, 22 and 21 shown inFIGURE 2 and into the annular chamber surrounding the liner.- Toincrease the cooling surface, radial chambers 41 can be provided withinthe liner 42. FIGURE 4 shows these chambers 41 and longitudinalpassageways 43 as they are disposed within the liner. If desired,particularly when liner 42 is constructed from a material with a highheat conductivity, chambers 41 can be omitted, whereby the centrallongitudinal passageway is cooled by conduction through the body ofliner 42. Although this embodiment is illustrated with sevenlongitudinal passageways it is, of course, apparent that the number aswell as the geometrical arrangement of these passageways can be variedas desired.

The embodiment shown in FIGURES 3 and 4 can also be constructed of fluidpermeable metal, glass, ceramic or graphite, the latter being preferredwhen heating solutions containing fluorine and phosphorous compounds.Circulation of a fluid through the annular chamber will then result inpermeation of the fluid through the liner body into the longitudinal gaspassageways 43 in the same manner as discussed in the embodiment ofFIGURE 2.

FIGURE 5 illustrates the embodiment of the invention which providesscraping means to remove solid deposits. This modification greatlysimplifies the nozzle construction since no internal fluid chambers areneeded for circulation of'a cooling or washing fluid. Instead the nozzlecomprises a simple solid member provided with a longitudinal flowpassage and adapted to fit on the end of the dip tube. A bob 46, whichis shown as a truncated cone in FIGURE 5, is placed upright within thenozzle flow passage, shown by the broken lines. A circular plate 45,which has a diameter greaterthan the nozzle discharge diameter, isattached to the base of cone 46 to prevent the bob from passing upwardlythrough the nozzle or from becoming wedged within the flow passage.Beneath the bob and attached to base plate 17 is placed a bob stop 44which restricts the downward movement of the bob. When in use, thenormal pressure fluctuations in the heater cause bob 46 to move up anddown against the nozzle surfaces, scraping off any deposits which mayform on this surface. In order to reduce eddy formation, the bob can beegg-shaped, i.e., having conoidal-shaped ends with the upper endsufiiciently small to permit entry of the bob into the nozzle flowpassage and the lower end sufficiently large to prevent the bob frompassing upwardly through the nozzle. Spiral grooves or a raised spiralthread can also be provided on the bob, whether it be egg orcone-shaped, so as to spin the bob and increase its scraping effect.

Bob 46 can also be mechanically moved up and down or about its axiswithin the nozzle flow passageway. To accomplish this, a shaft connectedat its upper end to plate 45 can extend downwardly through base plate 17and the bottom of vessel 15. This shaft can then be reciprocated orrotated to impart a suitable movement to plug 46. Bob 46 can also bemoved by installing an electromagnet within bob stop 44 and constructingthe bob of a magnetic material. A compression spring is then placedbeneath bob 46 to hold the latter against the nozzle flow passagewaywhen the spring is in its extended position. When an electric current issupplied to the electromagnet within stop 44 it will pull bob 46 down,compressing the spring. Stopping the current flow will release the boband the spring will force it up against the flow passageway, knockingofi any deposits on the surface of the passageway.

The apparatus shown in FIGURE 1 was employed to concentrate wet" processphosphoric acid. A ceramic burner encased in a stainless steel shell wassupported on the end of a gas supply conduit, which Was sufiicientlysmall inch in diameter) that the flame was prevented from moving up intothe supply conduit. The clip tube surrounding the burner was of one inchthick graphite having an internal diameter of about five inches andextending downwardly into the subjacent contacting vessel about 2 feet.The internal diameter of the subjacent contacting vessel was about seveninches, and the internal diameter of the upper disengaging vessel 8 wasabout twelve and one-half inches. During operation, a pre-mixed streamof natural gas and air was supplied to the burner at a constant rate toprovide a constant heat release to the acid, and the acid feed rate wascontrolled to maintain a constant acid temperature. The acid temperatureand residence time were chosen to produce a concentrated acid havingbetween about 67 and 73 percent P The stack gases from conduit 5 werescrubbed with a single countercurrent spray of water, and then vented tothe atmosphere. The gas was sampled after the water spray and the P 0content of this sample was found to be a reliable indication of theamount of phosphoric acid aerosols in the stack gases. The pressuredifferential between a point within the dip tube slightly above theburner and a point in the exhaust gas stream was recorded to indicatechanges in flow conditions through the system. The following exampleswill illustrate the invention:

EXAMPLE 1 The apparatus described above was employed without run periodof five hours, the dip tube was removed and found to be heavily coatedwith solid deposits in the vicinity of the burner. These deposits,formed when the acid splashed up into the dip tube and dried in place bythe hot gases, completely covered the narrow annulus between the outerstainless steel shield of the burner and the dip tube. The depositsadhered so tenaciously to the ceramic burner tip and steel shield thatthey had to be ground off.

EXAMPLE 2 To prevent acid from splashing up into the dip tube, a plugwith a single longitudinal passageway having a throat diameter of oneand five-sixteenths inches and smooth entrance and exit sections,similar to that shown in FIG- URE 2, was employed. No provisions weremade to wash or cool the plug, nor was the gas passageway sur facecoated with a water repellant material. The apparatus performedsatisfactorily at the start of the run, but after four and one-halfhours, the pressure drop across the dip tube and nozzle had increasedfrom 17 to more than 50 inches of water indicating the formation ofsolid deposits in the gas flow path, and a white fume, which was notremoved by the water spray, was visible in the exhaust gases, indicatingexcessive aerosol formation. After the run, the dip tube and nozzle wereinspected and, although the dip tube was free from deposits, the exitportion of the nozzle, immediately below the nozzle throat, was coveredwith an annular layer of solids which reduced the flow area through thenozzle to approximately one half its original value. Other operatingdata are reported in Table 1. The high aerosol content and visible fumeof the exhaust gas was due to the extreme turbulence and eddy currentscaused by the solid deposits which protruded into the gas stream.

EXAMPLE 3 During a six hour run, the pressure drop across the lower endof the dip tube and nozzle remained constant at 31 inches of water andat the termination of the run, no deposits were found on the nozzle ordip leg surfaces. The operating data are shown in Table 1.

EXAMPLE 4 The nozzle embodiment shown in FIGURE 2 was again employed,but unlike Example 3, surfaces 32 and 35 were not sealed and water waspermitted to permeate through the liner to wash the surface of thelongitudinal gas passageway. After seven hours of use, the pressure dropthrough the lower portion of the dip tube and the nozzle had remained atits original value of thirty-one inches of water, and no deposits werefound on the dip tube or nozzle. Other data are shown in Table 1.

EXAMPLE 5 A nozzle was constructed from a circular graphite plate byboring twelve holes, one-fourth inch in diameter, through the plate,arranged in a circular pattern. The holes were bored at a slight anglefrom the perpendicular so that when the plate was fitted onto the end ofthe dip tube, they were directed downwardly and outwardly from thecenterline of the tube. This nozzle was employed to concentratephosphoric acid under comparable conditions to those of Examples 1 to 4,as shown in Table 1. At the start of the run, the pressure drop throughthe lower portion of the dip tube and nozzle was three inches of water,and at the end of a two and one-half hour run, the pressure hadincreased to one hundred twenty inches.

Table 1 6. The apparatus of claim 1 wherein said longitudinal passagewayhas a smoothly converging entrance, a narrow throat and a smoothlydiverging exit.

Example 1 2 3 4 5 Fed Acid Concentration, Percent 5 s4 54 30.

z 5- v Type of Constrietion N one Single Passage Single Passage SinglePassage Plug with 12 Nozzle. Cooled, Coated Cooled, Washed I.D. Holes.

. Nozzle. Nozzle. Firing Rate, B.t.u./Hr 78,600-.. 200,000 189,000185,000 200,000. Acid Temperature, F 450 450. 41 Min 398. P105 Aerosolsin Exhaust Gas, p.p.m l2 3,400 W4 372 4,750. Pressure Drop, Dip Tube andNozzle,

Inches Water;

Start 25 17- 31 31 3 Finish 27 50 ll 31 120. Run Duration, Hour 4% ti 72%.

From the above examples it is apparent that the lower end of the diptube must be constricted to prevent liquid surging and splashing withinthe dip tube. To avoid the formation of objectionable amounts ofaerosols, this constriction must take a relatively low pressure drop,and when used to heat liquids which tend to deposit solids, means mustbe provided to remove these solids from or to prevent their accumulationon the flow constricting surface of the nozzle.

These needs are successfully provided by the nozzles of this inventionwhich are preferably trumpet, conoidal or conical in shape, and whichmust be provided with either a liquid repellant coating, washing,cooling or scraping means or any combination thereof to prevent solidaccumulation on the nozzle surface exposed to the hot combustion gasflow.

Having clearly and completely described the invention, we thereforeclaim:

1. An apparatus for the submerged combustion heating of liquids whichcomprises a vessel with liquid feed means and liquid withdrawal meansconnected thereto and means to remove products of combustion therefrom,a conduit extending vertically into said vessel, a burner having fueland oxidant supply means to produce a flame and generate combustiongases, said burnerbeing positioned within said conduit to discharge saidcombustion gases thereto, said conduit extending downwardly from saidburner a substantial distance, said distance being greater than thelength of said flame normally produced by said burner so as to preventthe flame produced by said burner from impinging onto said nozzle, theend of said conduit terminating within said vessel, beneath said liquidwithdrawal means, and having a.- discharge gas passageway comprising athroat of constricted cross-section and an exit section immediatelybeneath said throat, said exit section having asmoothly divergentcross-section so as to expand gases flowing therethrough and into saidvessel, an annular chamber surrounding said discharge gas passageway,said chamber positioned beneath said throat so as to surround only saidexit section of said nozzle and means to circulate a heat exchange fluidthrough said annular chamber to cool the surface of said exit section ofsaid discharge gas passageway.

2. The apparatus of claim 1 wherein said annular chamber is separatedfrom said longitudinal gas passageway by a common Wall of a materialwhich is permeable to fluids.

3. The apparatus of claim 1 wherein said discharge gas passagewaycomprises a nozzle having a removable liner sleeve, said sleeve formingsaid longitudinal gas passageway.

4. The apparatus of claim 1 wherein said longitudinal passageway iscoated with a water repellant coating.

5. The apparatus of claim 1 wherein said nozzle contains a plurality oflongitudinal passageways.

7. The apparatus of claim 6 wherein said entrance has a maximum angle ofconvergence no greater than about 50 degrees and said exit has a maximumangle of divergence no greater than about 40 degrees.

8. An apparatus for heating liquids comprising a first conduit having aclosed upper end and a nozzle at its lower end, said nozzle having acentrally disposed longitudinal passageway having a lesser diameter thansaid first conduit and a smoothly diverging exit portion with aninterior annular chamber surrounding said exit portion of saidpassageway along a substantial portion of its length, said chamber beingseparated from said passageway by a common wall, a gas burner disposedwithin said first conduit and positioned above said nozzle a distancegreater than the length of flame normally produced by said burner so asto avoid flame impingement on said nozzle, gas supply line to saidburner, a first vessel having a closed upper end concentricallysurrounding the lower portion of said first conduit; said firstvesselhaving a substantially greater internal diameter than the outsidediameter of said first conduit to form a first annular spacetherebetween; a second vessel closed at its lower end and having alesser diameter than said first vessel, said second vesselconcentrically surrounding said first conduit, disposed beneath and inopen communication with said first vessel and having a slightly greaterinternal diameter than said outside diameter of said first conduit toform a second, narrow annular space therebetween, means sealing theouter upper periphery of said second vessel to the inner periphery ofsaid first vessel; means to circulate a heat exchange fluid through saidinterior annular chamber, liquid feed means extending through the closedlower end of said second vessel, liquid withdrawal means in the lowerportion of said first annular space and vapor withdrawal means from theupper portion of said first annular space.

9. The apparatus of claim 8 wherein said longitudinal passageway has asmoothly converging entrance, a narrow throat and a smoothly divergingexit.

10. The apparatus of claim 9 wherein said entrance has a maximum angleof convergence no greater than about 50 degrees and said exit has amaximum angle of divergence no greater than about 40 degrees.

11. The apparatus of claim 8 wherein said common wall is composed of afluid-permeable material.

;12. The apparatus of claim 8 wherein said constricted lower end of saidfirst conduit comprises a short section having a plurality oflongitudinal passageways of lesser diameter than said first conduit,said section containing an annular chamber surrounding said passagewaysalong a substantial portion of their length, but separated therefrom,and fluid supply and withdrawal conduits communicating with saidchamber.

13. The apparatus of claim 12 wherein said chamber 1 1 1 2 is separatedfrom said longitudinal passageways by a flu 2,772,729 Mayhew Dec. 4,1956 permeable i l, 2,893,900 Machlin July 7, 1959 2,902,029 Hill Sept.1, 1959 References Cited in t11fil6 Of t Paffiflt 2,962,221 Kunz Nov,29, 1960 UNITED STATES PATENTS FOREIGN PATENTS 1,730,440 Smith Oct 8,1929 759,062 Great Britain Oct. 10, 1956

1. AN APPARATUS FOR THE SUBMERGED COMBUSTION HEATING OF LIQUIDS WHICHCOMPRISES A VESSEL WITH LIQUID FEED MEANS AND LIQUID WITHDRAWAL MEANSCONNECTED THERETO AND MEANS TO REMOVE PRODUCTS OF COMBUSTION THEREFROM,A CONDUIT EXTENDING VERTICALLY INTO SAID VESSEL, A BURNER HAVING FUELAND OXIDANT SUPPLY MEANS TO PRODUCE A FLAME AND GENERATE COMBUSTIONGASES, SAID BURNER BEING POSITIONED WITHIN SAID CONDUIT TO DISCHARGESAID COMBUSTION GASES THERETO, SAID CONDUIT EXTENDING DOWNWARDLY FROMSAID BURNER A SUBSTANTIAL DISTANCE, SAID DISTANCE BEING GREATER THAN THELENGTH OF SAID FLAME NORMALLY PRODUCED BY SAID BURNER SO AS TO PREVENTTHE FLAME PRODUCED BY SAID BURNER FROM IMPINGING ONTO SAID NOZZLE, THEEND OF SAID CONDUIT TERMINATING WITHIN SAID VESSEL, BENEATH SAID LIQUIDWITHDRAWAL MEANS, AND HAVING A DISCHARGE GAS PASSAGEWAY COMPRISING ATHROAT OF CONSTRICTED CROSS-SECTION AND AN EXIT SECTION IMMEDIATLEYBENEATH SAID THROAT, SAID EXIT SECTION HAVING A SMOOTHLY DIVERGENTCROSS-SECTION SO AS TO EXPAND GASSES FLOWING THERETHROUGH AND INTO SAIDVESSEL, AN ANNULAR CHAMBER SURROUNDING SAID DISCHARGE GAS PASSAGEWAY,SAID CHAMBER POSITIONED BENEATH SAID THROAT SO AS TO SURROUND ONLY SAIDEXIT SECTION OF SAID NOZZLE AND MEANS TO CIRCULATE A HEAT EXCHANGE FLUIDTHROUGH SAID ANNULAR CHAMBER TO COOL THE SURFACE OF SAID EXIT SECTION OFSAID DISCHARGE GAS PASSAGEWAY.